This invention relates to a cushioning material and a method for applying the same to a core-coil assembly of a resin cast transformer.
The basic building block of a transformer is the metal magnetic core. The core can generally be made out of a stack of metal laminations or sintered metal powder. The most common core shapes are rectangular and ring-like. In order to achieve a high degree of accuracy and efficiency in the finished transformer, it is important that the magnetic properties of the core are maintained throughout the manufacturing processes.
One type of material used in manufacturing transformer laminated cores, is grain oriented silicon steel. During the manufacturing process for the core, the grain of the steel is groomed as much as possible to flow in one direction. This is to allow optimum current sensitivity. By having the grain of the steel aligned in one direction, the maximum magnetic field loss is at its lowest value. With the magnetic field loss at its lowest level, the transformer's sensitivity to current flow is at its highest level, which means that the transformer has the highest response in current flow measurement.
When the transformer is assembled and packaged, an electrically insulating resin material is used to seal, that is, encapsulate, the components including the core and the coil wound thereon. The encapsulating resin provides electrical, mechanical and environmental protection to the core-coil assembly and allows safe handling of the transformer. The encapsulating resin is typically a thermoset polymer or resin, which is a polymer material that cures, through the addition of energy, to a stronger form. The energy may be in the form of heat (generally above 200 degrees Celsius), through a chemical reaction, or irradiation. A thermoset resin is usually liquid or malleable prior to curing, which permits the resin to be molded. When a thermoset resin cures, molecules in the resin cross-link, which causes the resin to harden. After curing, a thermoset resin cannot be remelted or remolded, without destroying its original characteristics. Thermoset resins include epoxies, malamines, phenolics and ureas.
When a thermoset resin cures, the resin typically shrinks. Because the resin surrounds the core, the shrinking thermoset resin exerts high mechanical stresses and strains on the grain oriented silicon steel core of the transformer. These stresses and strains distort the oriented grains and increase resistance to the magnetic flux flow in the laminations. This distortion and increased resistance results in higher core loss which causes the sensitivity of the transformer to decrease and diminishes the accuracy of the transformer. In addition, when the thermoset resin shrinks around a sharp protrusion, cracks typically form in the resin. The cracks may grow over time and compromise the seal that the resin provides to the internal components of the transformer.
Several prior art methods have been developed to protect a transformer core from the foregoing problems caused by shrinking resin. These methods include:                (a) wrapping the core with solid rubber cushions that are specially molded to fit snuggly around the core; and        (b) enclosing the core in a molded plastic “core cup” that isolates the core from the shrinking resin.        
However, both of the above methods of protecting the core from the stresses and strains arising from the shrinking resin are expensive since each core has to have a uniquely molded boot or cup. If there is a slight variation in the size of the core, the boot or cup does not properly fit around the core and thus the boot or cup provides ineffective protection.
It would therefore be desirable, to provide a transformer with an improved cushioning material which protects a core/coil assembly of the transformer from the stresses imparted by the shrinking of a thermoset resin used to encapsulate the core/coil assembly and which helps preserve the integrity of the thermoset resin. The present invention is directed to such a cushioning material and a method for applying the same to a core-coil assembly of a resin cast transformer.